This invention relates to recovery of oil from an interbedded, hydrocarbon bearing subterranean formation while electrically heating the producing strata. More specifically, this invention pertains to selective electrical resistance heating of a layered oil-bearing formation wherein power input and production outlet are controlled to selectively use the characteristics of the layered formation.
It has been proposed, for example, in U.S. Pat. Nos. 3,642,066; 3,874,450; 3,848,671; 3,948,319; 3,958,636; 4,010,799 and 4,084,637 to use electrical current to add heat to a subsurface pay zone containing tar sands or viscous oil to render the viscous hydrocarbon more flowable. Two electrodes are connected to an electrical power source and are positioned at spaced apart points in contact with the earth. Currents up to 1200 amperes are passed between the electrodes. The effectiveness of the electrical heating process depends on effective utilization of electrical power.
Certain formations, for example, the Ugnu formation in Alaska, are compresed of alternating relatively thin layers. Geologically such formations are called interbedded formations. In some interbedded formations, the conductivity of oil-bearing strata have an electrical conductivity which is much lower than the electrical conductivity of non-oil-bearing strata. For example, the Ugnu formation is comprised of alternating layers of sand containing oil and siltstone. The electrical conductivity of the siltstone layers is much higher than the electrical conductivity of the oil layers, for example, the siltstone conductivity may be ten times that of the oil-bearing sand. It is the primary object of this invention to provide a more efficient method of utilizing electrical power to apply heat to the oil-bearing layers of an interbedded formation.